text:
Why is the shape of the cargo compartment similar in large aircraft?
Why are the lower deck areas shaped like that, does that save some space? I find it common across lot of big aircraft.
Pic of a 777:

The answer lies in the existence of something called a Unit Load Device (ULD) -- basically a cargo pod for aircraft that's the equivalent of the shipping pallet.
ULDs allow oddly-shaped cargo (bags, assorted-size boxes, etc.) to be loaded efficiently in a standard volume and easily secured for flight (using hold-down lugs in the floor).
The shape of the cargo deck is designed to pack in as many ULDs as possible:

and as you can see from the cross-section the sloping sides on the bottom are a concession to the circular cross-section of the fuselage/pressure vessel.
Some other advantages of ULDs as they pertain to airline operations and baggage loading are described over on this question.

Why not expand the cargo compartment to use the whole bottom half?

There are a number of systems (e.g., power, flight controls, environmental ducts) routed through the spaces alongside & under the cargo area.

@immibis: Standardization. It's more economical to use a standard part for all aircraft than require each type of aircraft to use different containers. Conceivably, when ULDs were invented they use a higher % of the cross section of the average airliner. Nowdays widebodies are common so they end up using a smaller % of the cross-section.

@immibis I'm not sure what you mean by "the whole bottom half" -- are you saying just throw things into the bottom half-cylinder of the fuselage? If so, how will you accomplish the loading? (Hint: "Throw everything in there loose" doesn't work: It's inefficient in both space and time, and has other problems detailed in the post I linked to.) Similarly having half-cylinder ULDs doesn't work (they're too difficult to handle on the ground). ULDs trade a relatively small percentage of wasted space for enormous gains in cargo-handling efficiency.

@voretaq7 No. See the gap under the cargo bay in the photo, and the gaps to the sides - I meant expanding the cargo bay to use that space (and expanding the containers to fill the new cargo bay)

With airlines being cost-concious, I am sure they must be thinking of changing the design to accomodate more stuff... Good explanation none the less..

@immibis: The bottom surface holds rails for quick loading. That's intentionally a horizontal surface. The space to the side is fairly small.

Does anybody have a similar cross section image for the A320. Couldn't find anything good in google.


text:
Where are the locations of the elevators or horizontal stabilizers in a Concorde?
I don't find the horizontal stabilizers or the elevators of a Concorde at the tail section like other aircrafts. Are they fitted beside the ailerons in the wings?

There is no "horizontal stabilizer" on the Concorde like other commercial aircraft, the wing performs that duty because it is a delta-wing. The elevators for the Concorde are combined with the ailerons into something called "elevons" which perform both functions (Think stereotype paper airplane).
Note: Elevators and Horizontal stabilizers are not the same thing. Most aircraft have their elevators located on their horizontal stabilizers, only a few have the whole stabilizer as an elevator.
Source here

Are both the elevons rised or lowered at the same time in a Concorde to make it descend or ascend?

@DwiparnaDatta: Yes. Google "elevons". They're used often on jet fighters that don't have separate elevators. Moving the stick up and down make the elevons move in the same direction (down and up) like an elevator. Moving the stick left and right make the elevons move in opposite directions like ailerons. In a turn where you're controlling both aileron and elevators the elevons move slightly up and slightly in opposite directions.

source: Stack Exchange

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sort: votes


text:
Why are turboprop intakes placed below the propeller?
On many turboprop aircraft, such as the A400M and the ATR 72, it seems that the air intake tends to be placed below the propeller (images: Wikipedia):

source

source
Given that mounting the intake in a higher location helps reduce the risk of foreign object damage from ingesting debris into the core, why are the intakes still mounted below the propeller? Are the wings and engines already mounted high enough that the intake location doesn't matter when it comes to FOD, or does another issue (maybe accessibility for ground crews?) outweigh the risk of FOD?

Inertial separators are augmented by gravity when located under the engine. It is easier to discharge ingested ice and debris downward, rather than some other direction, so that is where they go if possible. Inertia keeps solid matter low at the indicated point while air makes the turn into the engine.

C130 Nacelle

@ymb1 You are correct, but in this location they are augmented by gravity as I said in my answer.

I believe the main reason is simply mechanical practicalities.
The bulk of the engine core is mounted overlapping the structural members of the wing, either below or above the wing spars.
The propellor is driven through a speed-reduction gearbox, which usually also offsets the propellor shaft from the turbine centreline.
That offset is used to place the propellor nearer the centreline of the wing to optimise airflow.
That in turn means the most direct air intake path to the turbine is the opposite side of the core to the propellor & gearbox.
See the photos below:

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text:
Why does the F-15 have a swept-back trailing edge at its wing tips?
The F-15's wing has an curious feature: a swept-back trailing edge near the wingtips, along with an unswept inboard portion of the wing.

(Modified from https://en.wikipedia.org/wiki/McDonnell_Douglas_F-15_Eagle)
This feature is not present on comparable teen-series fighters (e.g. the F-16):

(https://en.wikipedia.org/wiki/General_Dynamics_F-16_Fighting_Falcon)
Lots of Soviet-made aircraft (e.g. MiG-25, -29, and Su-27 families) appear to use a swept trailing edge. However, these fighters all have a constant sweep across the wing--not two sections of different sweep like the F-15.

(https://en.wikipedia.org/wiki/Sukhoi_Su-27)
Why then does the F-15 have such a seemingly unique feature (i.e. a trailing edge with two sections of differing sweep)? Clearly its designers must have thought it served some purpose--I presume they would not have added it for mere decoration.
EDIT: to clarify, my question seeks the physical reason(s) for why the trailing edge of the wing tip has a rearward sweep--not why the wings are cropped at an oblique angle, or a citation of historical precedent.
EDIT 2: I'd also like a well-sourced answer, if possible.

I've always noticed this but never thought about it long enough to actually ask why, so thanks for posting this

It looks like an attempt to to increase aspect ratio a little (compared with delta). F16 is a slower airplane and has less sweep angle, so the aspect ratio is a inherently lower. Flow separation is strong at high AoA so you don't want wing root to be too long (or aspect ratio too low) on a fighter, unless there's a vertex generator like canard wing strakes (which F16 has and F15 not), (not sure if the addition of vertex controller is reason between SU-27 and SU-57's trailing edge angle, maybe?)

@theunamedguy Those and the dogteeth on the Hstab appear to be an effort to improve high AOA performance.  Notice it also changed angle of aileron control surface slightly, perhaps in an effort to deflect turbulence away from the tail.  A longer wing tip chord would increase stall AOA.

Lots of planes have wings with swept trailing edges.  You might just as well ask why the inboard portion of the trailing edge of the F-15's wing is NOT swept, given that the outboard portion of the trailing edge of the F-15's wing IS swept.  Granted, trailing-edge sweep may be less common in a wing with a delta shape like the F-15's wing, but still--  Look at the planform view of the wing of the F-4 here.  I see a swept trailing-edge -- https://en.wikipedia.org/wiki/McDonnell_Douglas_F-4_Phantom_II#/media/File:McDONNELL_DOUGLAS_F-4_PHANTOM_II.png

@quietflyer That is a good point and is actually what I was looking for. I suppose my question is better stated as: why does the F-15's wing have a varying trailing-edge sweep through its span? I'll edit it to reflect this.

The F-15 wingtip is a "raked" tip configuration, which increases lift and decreases drag.1
[]2
This is the design decided by the USAF after discovery of several design deficiencies during initial flight test which included aeroelastic deficiencies (wing bending) and aerodynamic effects (buffeting). "In early June 1974, the Air Force flew the “raked” tip configuration and were completely satisfied
with the configuration’s buffet characteristics."  The Saga of the F-15 Wingtip

The actual answer is straightforward.  The F-15 wing was originally just a swept wing with a given taper ratio which resulted in a slightly swept trailing edge.  The inboard portion was straightened just to simplify the flap.  Because the flap hinge was now perpendicular to the fuselage the end of the flap was effectively sealed against the "booms" of the fuselage.  This increased the flap effectiveness significantly.
The wing tips were raked because the tips were developing more lift than expected and causing a slight overload on the wings. The rake reduced the wing area slightly and because of the "conical camber" built in the effective washout was increased as well which solved a airflow separation issue under high load conditions.  So 2 birds with one stone.
The above was from early books (1970's) published about the F-15.

"The rake reduced the wing area slightly and because of the "conical camber" built in the effective washout was increased as well "-- it would be great if it could be explained just a little more what "conical camber" is and how it relates to "effective washout".  Even just a link or two would help.  Appears to be great answer even just as is though.

Not sure about the hard core aerodynamics/physics you're trying to explain, but simply put, the curved wingtip doesn't "bleed" as much energy/airspeed in high AOA flight/turning fight, like the pure sharp Delta tip. Explained to me by F-15 test pilot.

You may develop your answer, at least by adding some brief explanation of why this phenomena is observed, and quite better by adding links to further detailed reading

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text:
Can a commercial pilot fly under this situation? about private carriage
scenario 1.
I'm a commercial pilot/flight instructor and employee of the flight school, who carries the flight school’s CEO and other staff on business trips, do you know what part of the regulations that fall under?
scenario 2.
If the CEO's office calls and asks you to pick up a donor and fly them to Newyork for a weekend of shopping,
is this part 91, 121, 135 or an exception under 119?

I'm going to pop some popcorn and watch this one...  ;)

#1 is company owned plane carrying company people on company business.  Sounds 91 unless there is more to it.  #2 is a charter.

Questions on this site are better focusing on 1 question each - otherwise you get a partial answers as has already been demonstrated. You may like to edit this to focus on q1 which has an answer, and repost q2 as a separate question.

Both scenarios appear to be under 14 CFR Part 91. Scenario #1 is private carriage by virtue of being an employer owned aircraft flying employees, flown by an employee, for company business. Scenario #2 can get sticky. But, if the “donor” is not paying for the flight or compensating you or the owner/operator, it would still be Part 91. However, the “donations” would have to be justified.

#2 requires common purpose and hours are considered compensation.

@acpilot - The original question from Soartosky is about a person with a Commercial Pilot Certificate. Commercial and ATP pilots can be compensated under Part 91. In this case, the pilot would already be getting compensation from their employer, the owner/operator. The question would be whether the owner/operator can allow this operation under Part 91 considering the “donor” presumably has made contributions to some other part of the business not related to the flight department. For instance, is the donor the owner of a leaseback aircraft?

You're right.  I'm conditioned to read all of these as PPL questions.

this is my think flow.
#1 scenario.
first, let's look at each person's role.
CEO is operational control of flight because we use CEO's aircraft.

FAR says: operational control, with respect to a flight, means the exercise of authority over initiating, conducting or terminating a flight.

in addition, CEO is also not a commercial operator because business trip is merely incidental to other business and it is, in itself, not a major enterprise for profit.
So, CEO doesn't need a operating certificate.
I'm a hired pilot and have commercial certificate.

FAR 61.133 says:

(a) Privileges -
(1) General. A person who holds a commercial pilot certificate may act as pilot in command of an aircraft -
(i) Carrying persons or property for compensation or hire, provided the person is qualified in accordance with this part and with the applicable parts of this chapter that apply to the operation; and
(ii) For compensation or hire, provided the person is qualified in accordance with this part and with the applicable parts of this chapter that apply to the operation.

So, I think it just exercise my commercial pilot privilege.
and If I have a instrument rating in same category, class and type and meet part 91 medical etc.
I think I can go and I will fly under part 91.
welcome any opinion about that answer :) and I will write for #2 scenario later.

Scenario 1 would likely be private carriage according to the AC 120-12, because you're carrying a limited number of people from a defined group.
Number 2 we need more info. It either falls under charter/135 or fractional/91 or might be a contract if he is the only person you fly for.

He is the only person I fly for. and both #1 and #2 scenario use school's airplane.


text:
Why is the RPM limit on a Lycoming IO-540 different in the -C and -D versions?
On a Lycoming IO-540, it seems there is a Max RPM difference between the -C and -D versions (and thus a max HP difference too).  Does anybody know what is different in the engines that causes this, or are they similar internals with a different governer?

Those engines have different valve configurations, which mean they "breathe" differently. Better breathing supports more RPM's before the engine runs out of puff, so the most probable cause is the better breathing in the one that revs higher.

What are the different configurations?  Both are parallel valve.

I thought one of them had the angle valve heads.


text:
Is this test question correct about the causes of detonation?
So detonation is explained brilliantly in the answer of this question. In the accepted answer, NathanG goes on to say the following:

the octane rating of a fuel is basically the amount of pressure it can take before it detonates

From what I understand, the higher the number on the octane value, the more pressure the fuel can take, which is why it is used in engines with higher compression ratios.
Assuming octane works exactly the same way in aircraft piston engines as in cars, this has left me confused about a question I was faced with on a small test:

Which of the following situations may lead to detonation in an aircraft engine?
A: Running the engine at too rich a mixture setting
B: Running the engine at too lean a mixture setting
C: Using fuel of too high an octane rating
D: Using fuel of too low an octane rating

I thought the correct answers to this were B and D, yet the correct answers were apparently A and C. Can someone explain why this is the case? I'm just not seeing it.

We are not seeing that either. This question is not about piston engines, it's more about how to find trustworthy information.

I think that 'detonation' as used in the question has a different meaning to the one you started off with. Taking your initial meaning, if detonation is not happening then the engine is not running. That (initial) meaning renders the question nonsensical, so the 'detonation' in the question must be talking about something else. Think about what 'back-firing' is.

"if detonation is not happening then the engine is not running" - I think you're referring to combustion there.

@simonatrcl What makes you think that hiigaran was mistaken about the meaning of the word "detonation"? I don't see any evidence of that.

You are correct, the answer is B and D. Another common error is warning signs that say "danger high octane fuel". They should read, "danger LOW octane fuel!"*.

So the examiner probably just made a mistake when marking, then?

Yes, the writers of test questions are human too.  It would appear B and D are the correct answers.
According to the Lycoming website, for pilots, the greatest danger for detonation in aircraft engines is too lean a mixture.  Running rich at full throttle keeps cylinders cooler. Answer B is correct. Excessively lean mixtures can also be caused by clogged fuel injectors or air intake leaks.
Lower octane also increases risk of detonation because it's "autoignition" temperature is lower than higher octane fuel.  Autoignition can happen without spark due to higher temperature and pressure.  This throws the "timing" of piston power stroke and ignition off, causing a "ping" or "knock".  Engine damage may result. Answer D is correct.
So running rich at high power settings, while producing slightly less peak power, greatly extends the life of your all important engine.

Forgot to go back and accept this answer. If anyone is interested, the instructor who graded that test was fired after clipping a wing on a fence (and two aircraft on two previous occasions).

Might be the eyes.  Mine are not as good as they used to be.


text:
How do I calculate the point of no return for a flight with multiple legs?
In every textbook where Points of No Return (PNR) are discussed (or at least, those I've read), the method of calculating them is always between point A to point B as a direct route. This is simple enough to do with the appropriate formula.
What I've never come across is an application of this in a more realistic scenario, where an aircraft will have at least one waypoint along the way, or a change in altitude/flight level, where the wind conditions or a change in TAS result in a change of ground speed.
Take the following example:

Leg
Track
Ground Speed
Wind
Distance

A -> B
000
100
090/50
50

B -> C
270
150
090/50
25

What's the PNR for A -> C?

I spent several minutes wondering what it was I forgot when I submitted the question. That was embarrassing. Edited.

On the return do you need to backtrack the original legs or can you go straight?

The point of no return is defined as the point where the travel time to the destination (C) is equal to the travel time when returning to the departure (A). On each leg, calculating travel times is trivial with the known ground speed and distance ($ \text{time} = \text{distance} / \text{speed}$). In our example we would get the following for the time to get to point C as a function of distance $d$:
$$ \text{time to C} = \begin{cases} (50 - d) / 100 + 25 / 150, & d \leq 50 \\ (75 - d) / 150, & d > 50 \end{cases} $$
When assuming a ground speed of 50 for C -> B (tailwind turned into headwind), a similar equation can be created for the return case:
$$ \text{time to A} = \begin{cases} d / 100, & d \leq 50 \\ (d - 50) / 50 + 50 / 100, & d > 50 \end{cases} $$
Plotting the two functions, we get this picture:

The distance where the two time curves intersect, is the point of no return. It can be calculated by equating the two straight line equations for the blue and red curve:
$$ \frac{50 - d}{100} + \frac{25}{150} \stackrel{!}{=} \frac{d}{100} \quad \Leftrightarrow \quad d = \frac{100}{3} \approx 33.3 $$

Would graphs of fuel required to reach points A and C give the same final result?

(Maybe grounds for a new ASE question?)

@quietflyer Sure, fuel flow per time is roughly constant (slightly decreasing due to the lowering weight), but it does not depend on head- or tailwind. If you fly at different altitude to avoid the headwind, things start to get more interesting...

Is there a formula you use to calculate that intersection? I'm looking to see if I can build a spreadsheet that calculates a lot of my flight planning. Got everything made, except this.

@hiigaran I added the formulas for the lines and how the PNR was calculated. They should be easy to generalize for more legs.

Cheers. I think that should be sufficient to work with. I think the limitation now is my tablet's ability to process all the data without locking up!

Isn't this answer about ETP instead of asked PNR? What am I missing?

The Point of no Return (PNR) is the furthest distance an aircraft can travel from it's departure point and return to this same departure point with the consumption of all usable fuel except fuel reserves ('Safe Endurance(SE)').

There are many practical applications where the journey out and the journey home have different true air speed values and fuel consumption rates (for example, one engine inoperative on the return journey). Since your question doesn't reference fuel consumption rates I am going to go ahead and assume the same fuel consumption throughout for the purpose of the PNR calculation.
Since we need to return home, the distance out ($d_{1}$) is equal to the distance home  ($d_{2}$) so lets just call this $d$
$$d = d_{1} = d_{2}$$
The safe endurance is the total time spent on the journey, that is the time out plus the time home.
$$ SE = t_{1} + t_{2} $$
Furthermore, by rearranging the basic speed equation $ s = \frac{d}{t}$ for time $ t = \frac{d}{s}$ we can say:
$$ SE = \frac{d_{1}}{O} + \frac{d_{2}}{H} $$
Where $O$ is the ground speed for the journey out and $H$ is the ground speed for the journey home.
We already know that $d = d_{1} = d_{2}$ so we can factor out $d$
$$SE = d \left( \frac{1}{O} + \frac{1}{H}  \right) $$
And with some basic algebra...
$$SE = d \left( \frac{O+H} {O\cdot H}  \right) $$
and multiplying each side by the inverse of the fraction to isolate distance to the PNR.
$$d_{toPNR} = SE \left( \frac{O\cdot H}{O+H}   \right) $$
Now lets consider a multi-leg journey

Firstly, consider the endurance to travel leg one (A to B) and return to the departure point (A)
$$Endurance_{A-B-A} = E_{1}= d_{1} \left( \frac{O_{1}+H_{1}} {O_{1}\cdot H_{1}}  \right) $$
The remaining SE avaible for the second leg (B-C) is the original (total) SE minus the endurance if the PNR was just leg 1 $E_{1}$ (that is, the endurance for the out and home journey of leg 1).
$$SE_{2} = \left( SE \right)_{Total} - E_{1} $$
Note, If $E_{1} > \left( SE \right)_{Total}$ then the PNR lies within the first leg. If $E_{1} = \left( SE \right)_{Total}$ then B is the PRN point.
$$d_{toPNR from B} = SE_{2} \left( \frac{O_{2}\cdot H_{2}}{O_{2}+H_{2}}   \right) $$
You could continue this process for any consecutive legs, by determining at which leg the the PNR endurance elapses the safe endurance.
To convert this into a time to the PNR from the departure, calculate and sum the times for each leg up to the leg on which the PNR occurs. Then calculate time to PNR for that leg (as a single leg question). Again, rearranging the speed equation $ s = \frac{d}{t}$ thus, $t = \frac{d}{s}$
$$t_{toPNR} =\frac{d_{toPNR}}{O}$$
Substituting in the $d_{toPNR}$ equation from earlier:
$$t_{toPNR} = SE \left( \frac{H}{O+H}   \right) $$


text:
Can I get a pilot's license if my driver's license has been suspended?
My driver's license is suspended because I got a bunch of tickets. It was the first time and I've paid the tickets. Can I still go for a private pilot's license?

The FAA has a guide on applying for a medical/student license
https://medxpress.faa.gov/MedXpress/Content/Docs/MedXPressUsersGuide.pdf
Near the end, pages 30-33. They ask for consent to review your National Driver Register.  Read those pages, see if any of it applies to you.

Can you include some of the relevant content so that those of us who hate clicking links can view it?

Unable. Cannot copy the PDF document.

Fair enough.   I should have clicked on the link and observed that!

I don't see how this answers the question? The MedXpress instructions are just instructions for filling out the application, they don't say anything about what may be disqualifying or not.

Read the box at the bottom of Block 18, and 20, on page 33, where it inquires about driving history. I don't know how much you have had to screw up driving to be disqualified for flying - but you are required to consent to them looking into it.  There's also this for more info https://www.faa.gov/news/updates/?newsId=87125   I was not able to locate anything with this search  https://www.faa.gov/search/?q=driving+record+disqualification  As far as I know, applications go to FAA's OK City office for review for anything that might be questionable.

More details  18.v. History of Arrest(s), Conviction(s), and/or Administrative Action(s).
Arrest(s), conviction(s) and/or administrative action(s) affecting driving privileges may raise questions about the applicant's fitness for certification and may be cause for disqualification. A single driving while intoxicated (DWI) arrest, conviction and/or administrative action usually is not cause for denial provided there are no other instances or indications of substance dependence or abuse. See Substances of
Dependence/Abuse.

From here  2018 GUIDE FOR AVIATION MEDICAL EXAMINERS
https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/aam/ame/guide/media/guide.pdf  Also discussed is the Sport Pilot license, with has limitations on what can be flown and where.  Ok for a start to flying. Can't do everything can be done with a Private Pilot License - that discussion can be left to another question.  Bottom line: keep your driving record clean to avoid giving the FAA any reasons to
go after your license.


text:
Can spar stresses be calculated using zero (wing) fuel?
For a homebuilt prototype aircraft meant for the primary or Canadian JAR-VLA category, is it OK to assume that spar stresses (other than landing loads) can be calculated using zero-wing-fuel numbers? Or perhaps the landing loads usually negate that zero-fuel stress relief?
So, when specifying plus and minus stresses, both the wing and its fuel content can be discounted except for full-fuel aileron loads?

In general, combining an empty wing with a full fuselage will give the highest stresses in the wing spar at positive and negative load factors. If the wing tanks held fuel, the weight of the fuel mass would reduce the wing spar bending moment inboard of the tanks at the same load factor.
However, details depend on the particular configuration. Make also sure you look at the most forward c.g. location and at the added load from aileron deflections.
Also, don't assume that the critical load case of one design will also be the critical of another. Please look at cases in all corners of the envelope to make sure you do catch the critical one.

When in doubt run the calculations at each extreme and take the worst result. (This being the 21st century the actual calculating is relatively fast thanks to modern computing power.)

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text:
What is this stripe on the windshield of the Embraer E-Jet?
I noticed that there is a stripe along the upper edge of both forward windshields in the Embraer E-Jets:

(cropped from Wikimedia)
I tried to find a better picture, but couldn't find anything closer. This YouTube video clearly shows the stripe several times:

What is this stripe used for? The only thing I can think of is window heating, but these elements usually look quite different, as can be here: What is this pattern on the A350 front window?

It's a heater bus bar. In fact, the Q/A about the sensor loops that you linked has a great example of the same if you scroll to the bottom of the best answer.

So there should be another of these at the bottom (probably hidden behind the glareshield) and then current flows between them for heating?

@Bianfable Of course you're right, in fact you can see them from the outside: https://i.ytimg.com/vi/HEWumjfD6w4/maxresdefault.jpg

Having never been in the cockpit of an Embraer E-Jet, I would have guessed it was a rod to move the sun shade left to right on  to adjust for the angle of the sun.


text:
What are the ASTERIX CAT010 message types 005 and higher?
I am analyzing some surface radar recording data under Asterix CAT010 category. The document from Eurocontrol shows that there are 4 standardized message formats under this category:

001 Target Report
002 Start of Update Cycle
003 Periodic Status Message
004 Event-triggered Status Message

However, I could find some other types from 005 to even a few hundred.
May I know what are these message types referring to?

What kind of ASTERIX decoder are you using? Where does the data come from, who is the manufacturer of the SMR, which radar model? Can you add a few messages (in hexadecimal notiation) so I can have a look?

Obelix? OBELIX!?

@DeltaLima Thanks for your suggestion. Finally, the issue is solved. I got a help from the author of this repository. https://github.com/CroatiaControlLtd/asterix There is some issue previously when decoding the CAT10 data. Now it is corrected.

@Peter It is a batch of data following the Asterix format from Eurocontrol.

Finally, it is solved. It is not about the data issue but some decoding issue. I am using an asterix-decoder in python from https://github.com/CroatiaControlLtd/asterix.
Based on the help of its author, Damir Salantic. "There was a bug in the presentation of hexdata in case when the old format of asterix is used. Old format has several Data Records in Data Block."
Now the issue is fixed and the decoder repository is updated the decoded data is following the format from Eurocontrol.

Hi Yicheng, thank you for taking the time to come back here and answering your own question.


text:
24 hour access at general aviation airport
I was doing a night flight with my kids and one had to go to the bathroom.  I was happy that when I landing at an airport (KVLA) and they had a door code so I could get to the bathroom.  Is there a list anywhere of small GA airports that you can get access to the bathrooms 24 hours a day?
If not, what is the best way to make one?

We use water bottle for boys and diaper for girls when we go on a trip.

Welcome to aviation.SE! If you're asking if GA airports are typically open 24x7, we already have this question. If you're asking more specifically about how to find airports that have toilets available 24x7 that's unlikely to be something we can answer here. That could change from day to day, especially given the changing COVID situation and restrictions in different parts of the US. The best way to find out if an airport has a specific service available is usually to just call the FBO there.

At night, once the airport is closed, they all have 24X7 access to toileting facilities as long as there is no landing restrictions. It is the big green latrine that runs alongside the runway.   While I appreciate the convenience that a bathroom in the FOB might afford, with the right amount of lighting (or lack thereof), I am not above assisting the airfield in its irrigation. Fertilization, on the other hand, is a different story. Stick to Class D and fuel available airfields. They are your best bet for an accessible FBO or pilot shack.

@DeanF. Getting arrested for Indecent Exposure can get you put on the sex offender registry in some places in the US. Suggesting that public urination is okay is irresponsible.

@nick012000: Way out by the runway is hardly public.

@nick012000 - You are absolutely right. Public urination is wrong. You should not urinate in a crowd. But, there is a huge difference between relieving yourself in a few acres of grassy area a half mile away from the closest building and peeing on the building itself. I did say at night after closing after all. Have you never gone camping, fishing, hunting, farming, ranching, off-roading, etc. All these activities occasionally include outdoor peeing. Also, before 6 and after 60, you are going to have to make peeing a priority. And, public urination is a citation, not an arrest in most places.

@nick012000 - Now, peeing in a stadium men’s room during the post season. That’s public urination. . But, on a serious note, the OP was asking about his kids. Out of all the police officers I know, not one of them is going to write a citation, arrest, or register a child as a sex offender for having to pee. Most of them are parents. And, most parents have had the experience of pulling onto the shoulder of the highway because Little Johnny could not hold it any longer. Eventually, it will get to the point that Big John can’t hold it either. If they are lucky enough to reach that age.

This devolved fairly quickly.  But anyway, Thank you for your response @Pondlife.  If I would have know that my kid was going to say she needed to go to the bathroom before we left then I would have called. But alas, she is 6.  It would be great to be able to plan rest stops over a 3 hour flight without having to call that many FBOs.

@Steven You're welcome, and it's a completely reasonable question. I just don't think there's a good, reliable answer.

As someone already commented, creating and maintaining such a list would be difficult, considering there are over 5,000 FBOs in the US alone. Proper flight planning is the best course of action if you know you are flying off-hours. Identify potential bathroom stops along your route, call the FBOs at those airports and ask.


text:
Why does CFR §91.215 part (b) list specific areas for transponder operations?
Why does part (b) list individual sections where transponder operations are required when part (c) states that transponder operation is required in all controlled airspace? All sections listed in part (b) are part of controlled airspace anyhow? Similarly, the AIM says that transponder operation is required the moment the aircraft starts moving until the time it stops moving (a simplified interpretation).
Am I missing some subtleties?
Details:
CFR §91.215:
Part (b) contains sections that specify precisely where transponder operation is required. For example:

(1) All aircraft. In Class A, Class B, and Class C airspace areas;
(2) All aircraft. In all airspace within 30 nautical miles of an
airport listed in appendix D, section 1 of this part from the surface
upward to 10,000 feet MSL;
(4) All aircraft in all airspace above the ceiling and within the
lateral boundaries of a Class B or Class C airspace area designated
for an airport upward to 10,000 feet MSL; and
(5) (i) In all airspace of the 48 contiguous states and the District
of Columbia at and above 10,000 feet MSL, excluding the airspace at
and below 2,500 feet above the surface; and
(5) (ii) In the airspace from the surface to 10,000 feet MSL within a
10-nautical-mile radius of any airport listed in appendix D, section 2
of this part, excluding the airspace below 1,200 feet outside of the
lateral boundaries of the surface area of the airspace designated for
that airport.

Then part (c) says:

(c) Transponder-on operation. While in the airspace as specified in
paragraph (b) of this section or in all controlled airspace, each
person operating an aircraft equipped with an operable ATC transponder
maintained in accordance with §91.413 of this part shall operate the
transponder, including Mode C equipment if installed, and shall reply
on the appropriate code or as assigned by ATC, unless otherwise
directed by ATC when transmitting would jeopardize the safe execution
of air traffic control functions.

The AIM says:
AIM Section 1:

4−1−20. Transponder Operation a. General

Transponder and ADS-B operations on the ground. Civil and military aircraft should operate with the transponder in the altitude
reporting mode (consult the aircraft's flight manual to determine the
specific transponder position to enable altitude reporting) and ADS-B
Out transmissions enabled at all airports, any time the aircraft is
positioned on any portion of the airport movement area. This includes
all defined taxiways and runways.

Transponder and ADSB Operations While Airborne. Unless otherwise requested by ATC, aircraft equipped with an ATC transponder
maintained in accordance with 14 CFR Section 91.413 MUST operate with
this equipment on the appropriate Mode 3/A code, or other code as
assigned by ATC, and with altitude reporting enabled whenever in
controlled airspace.

I know I’ve seen this question before, but I can’t find it. Short version is that B is negative (you can’t fly here if you don’t have it) whereas C is positive (once you have it, you must use it everywhere).

Aircrafts are not even required to have electrical systems for Day VFR in some airspaces....

I think what you're missing is that not all aircraft have transponders. The key words from 91.215(c) are (emphasis mine):

[...] each person operating an aircraft equipped with an operable ATC
transponder maintained [...]

To put it another way, (b) says "in order to fly in these areas you must have a transponder". Whereas (c) says "if you have a transponder, this is when it needs to be on".


text:
How often is communication lost with aircraft?
I just read the NTSB press release of Northwest Airlines Flight 188 that flew more than one hour without making any radio contact. I imagine there may be many causes for losing radio contact with the ground (distraction, electrical failure,...) and I found a video (in French) saying that one way to solve the situation is to send a fighter to have a closer view of the situation and possibly assist the pilots.
I wonder if the situation of losing contact with an airplane is frequent? I imagine it happens between one and ten times per month over a region as big as the USA, but that is just a guess.

I rephrased your question slightly because it looks like you're asking about lost communications rather than aircraft that are deliberately flying without radios. "NORDO" could be taken to mean both of those things.

OK, I thought "communication loss" and "NORDO" were synonyms

In some places radio coverage is just poor at lower altitudes too.  I've lost comm on the ATC specified frequency after getting a descent still 15 minutes out from my approach.  In this case I looked up the other frequencies in the area and was able to re-establish contact on an alternate.

According to this FAA paper, Table 3, ASRS reports were analyzed for 16 months between 1978 and 1979. This included 553 incidents where the recipient was not monitoring, which seems to be the closest category to loss of communications. This equates to a little more than 1 incident a day. Note that this only includes incidents that were reported, and air traffic has increased since then.
This report contains info from ICAO in in Europe. The focus is on the security issues from unresponsive aircraft. In just the northern region, there were 230 flights over just 6 months in the beginning of 2004. This number inclues airlines, business jets, and military transports. This frequency is a little higher than the previous report, but is more recent and probably includes less flights.
Other reports like this one don't include a time frame for the reports analyzed, but look at other factors involved. For example, pilots with fewer hours and GA pilots are more likely to be involved in such an incident. The average duration of these incidents was 7.6 minutes.
This page also has some good information about this type of incident, including a list of some of the many possible reasons for lost communications and excerpts from some of the reports.

An average ot just 7.6 mins sounds like user error, e.g. dialing in the wrong frequency or being distracted. That happens all the time, and I'd bet most cases aren't even reported. Any data on permanent loss of comms, e.g. equipment failure?

I think it happens all the time actually. Most of the time its likely not reported. When I did my first solo cross country in a 152, my radio button went out while using VFR flight following. Perhaps they did report it, but they likely just thought it was a new pilot not knowing what he was doing. It was really distracting for myself at the time as a pilot in training. I could hear them talking about me and it took a while to understand that they were not hearing my responses. As a pilot, you are supposed to squawk 7600 to let them know you lost your radio. At the time though, I couldn't remember if it was 7500 or 7600. I knew one of them was for the radio and the other was for a highjacking, and I wasn't needing a fighter jet for my broken radio.

I think losing contact for an hour is unusual, but not being in contact with ATC for a few minutes is pretty common.  On a cross the US flight you might hear 5-6 incidents.  Most of these seem to occur as planes are handed off from different sectors that use different frequencies.   It not uncommon for the pilot to tune in the incorrect frequency.
Also, as aircraft descend into fields with obstacles (ex. mountains) it is not unusual to lose contact.    As you fly along you sometime hear ATC attempting to contact a plane and get no reply.  If you have just heard that call sign you might volunteer to give the aircraft a call and then act as a relay.  For example, you are over some place and hear
ATC: "KingAir 123 contact center on 123.55"
KA123:   no response.
ATC: "KingAir 123 contact center on 123.55"
KA123:   no response.
ATC: "KingAir 123 over someplace contact center on 123.55"
You: "Center, N12345, we are over someplace, would like us to try to contact KA 123?"
ATC Center: "N12345, affirmative, contact KingAir 123 and ask them contact center on 123.55"
You: "KingAir 123, N12345, center would like you to contact them on 123.55"
KA123: "Contact Center 123.55, thanks N12345"

it's also possible you don't hear a reply because the plane is on the other side of the antenna and out of your range but inside ATC's range

In that case, @ratchetfreak, it would be like being in the same room with someone on a phone conversation - you'd hear 1/2 of it and know that communication was happening, just not the whole conversation.

A supervisor at the local ATC facility (medium sized TRACON/Tower) said they have loss of radio several times a month.  Sometimes the plane "lost" the frequency, and showed up elsewhere.  Sometimes they had an equipment failure.  On nice weekends, they have more, and those are written off as VFR pilot error situations.
While we were talking, I asked how many 7600 squawks they got per month, and he said somewhere between 0 and 2.  More common during the summer when more people are flying, and during the hard part of winter when there are just more equipment failures.
Not a big study, but just one ATC supervisor's opinion and observation.
As for looking at 1970 and 1980 data, the whole business of radios, and how well they work has changed quite a bit in 20 years.  I can remember radios being out as much as 1/3 of the time (well one or the other most of the time) back in the 70's.  Today it rarely happens.  For one thing, the radios today don't have the large ganged switches that the older radios had.  And the avionics guys say that today's transponders are more stable, and last longer as well.


text:
Do fighter jet pilots ever use autopilot?
I know that military jets have autopilot, but do the pilots actually prefer to use it?
I ask this question because, unlike civilian airplanes, military jets are more likely to come in a situation where the pilot may have to take quick decision and leave the designated path for some mission (e.g. to intercept a civilian jet flying on a wrong path). In such case, the autopilot may become a problem for quick maneuvering.

I would think they do for sure since usually it takes some time to fly to their mission zone and I highly doubt they keep their hands on the "steering wheel" all of the time. The pilots would get some kind of a transmission instructing them to divert to a different mission/path. They would have more than enough time to switch it off during the transmission or just after it.

Also note that modern military aircraft (particularly fighter aircraft) have extremely advanced onboard computers that calculate and perform operations that civilian autopilots would be completely jealous of. While much of the 8M+ lines of code are war-fighting specific, the goal of military autopilots is to free the war-fighter of remedial tasks so they can focus on their unique mission.

The Navy has 'auto pilot' for landing on carriers at sea. Its use is optional. Apparently letting the computer land for you is stressful for some aviators.

Some military jets such as the F-15 are inherently unstable by design (this is intentional to have good maneuverability). Flight computers (independent from autopilot) are always making sure the plane doesn't go tail over nose in supersonic regime, where it's borderline impossible for a pilot to control solely manually.

@shortstheory, F-15 is not statically unstable. F-16 is. Besides, at supersonic they are all stable (in pitch): the aerodynamic center shifts back significantly in this regime.

I'm not in the military, so you could say I'm talking out my rear end here, but based on my experience as a pilot: yes, military pilots use the autopilot all the time.
Here's my reasoning, in order of most to least convincing evidence:

Punching the autopilot off takes about a fifth of a second.  The button is right there on the yoke, for crying out loud.  The time it takes to turn it off is not a good reason not to have it on.
The autopilot is a labor-saving device.  When you're in an aircraft (and particularly when you're solo in one) there are times when you'd rather be paying attention to your chart or setting up the FMS or stretching or eating a candy bar instead of actually flying the plane. Also, when Air Force pilots fly long repositoning flights, the autopilot reduces fatigue between in-flight refuelings (the Navy uses carriers).
Times when you might have to leave the designated path for a mission happen a lot less than you think.  Again, not in the military, I'm just basing this on the realities of fuel and the number of military aircraft out there.  Maybe if you're flying close air support for teams on the ground or something, but remember that the military is first and foremost a logistics organization - it's a machine for moving people and things from one place to another.  Most of the time, military aircraft aren't on a hair-trigger moment's notice ready-to-engage posture, they're going from one airfield to another for training or maintenance or something.
In most aircraft, you can override the autopilot if you need to anyway.  Again, never flown a military fighter aircraft, but based on what I know about flying in general, even if you're chilling in your F-22 (with the autopilot on) if you grab the stick and yank, the airplane will move.  (I would be interested to know whether this is true of Airbus aircraft. EDIT: Question answered after 4 years!)

As far as I'm aware it's not true for an Airbus - you have to press the over-ride thumb button to take control (and even then, you're still reined in by the "Normal/Alternate Law" systems... which sound a lot like something out of The Terminator)

+1 for understanding that most of the military is the "shaft" (logistics), not the "tip" (combat) of the "spear."

My ROTC commander was the commander of the T-Birds for two years.  He said that they would trim the rudder almost full-forward, so to maintain straight and lever they needed to maintain about 25lb on the stick.  (Just thought about that when you mentioned labor saving.)

I would be interested to know whether you can override autopilot by yanking the stick on F-16 and newer including F-22 actually. They are fly-by-wire, the yoke is spring-loaded with very strong springs and only moves a little (in the early F-16 it didn't move at all, only sensed force, but it felt too weird, so it was allowed to move a little) and it generally behaves much like the Airbus normal law. So it might well require pressing the override button too (which still is on the stick; fighters have almost all control on the stick and throttle).

@JonStory: The normal law is basically just autotrim. The aircraft responds immediately to any input, just keeps itself trimmed so that when you release the stick it maintains attitude (actually roll and wing loading so it will pitch up when you slow down and vice versa).

@JanHudec 5 pounds of stick input on the super hornet will disengage the autopilot.  I imagine its very similar in other aircraft like the F22.

Airbus' autopilots can be disconnected by just touching and moving the sidestick out of the neutral position. Myself triggered the cavalry sound, once on a long flight to the canaries, when my upper leg shortly hit the controls... Regarding the handover switch, it's just there to indicate the system whose input has priority (pressed for at least 40sec for a successful handover). Otherwise the system adds both inputs being made.

@LRT Forty seconds?!?  That's an eternity!

@SteveV. You're right, feels infinite but that's what even the FCOM agrees to. After that period there is no mixing of both sidestick inputs anymore.

So you don't fly airbus, you don't fly military...what do you fly?

Military pilots are not unlike most other pilots: we're lazy and love booze, and we become just as autopilot crippled as the next guy.
Fighter aircraft controls are designed around the HOTAS philosophy, or hands on throttle and stick.  To quickly perform flight critical functions, like disengaging the autopilot, the pilot's hands do not even have to move from the controls.  On the Super Hornet the preferred method to disengage the autopilot is to paddle it off with the pinky finger on the stick.  However, 5 pounds of pressure on the stick will also disengage the autopilot.  Furthermore, some autopilot functions of the super hornet are always on.  For instance, the jet constantly trims itself to 1 G and this autotrim will relatively help maintain a stable platform.
Aside from laziness, there are times when the autopilot can be a large asset to the mission.  The cockpit of a fighter aircraft can get very busy when transitioning between phases of flight.
Here are a few mission related examples off the top of my head when having some form of autopilot is really nice:

When leading another aircraft (it makes it easy for the wingman if you are stable)
When flying parade and trying to do anything besides flying parade.  Flying parade sucks.
Configuring mission related instruments
Performing coupled approaches to the boat (scary)

There are as many, if not more, reasons to use the autopilot in a fighter aircraft than in our civilian counterparts.  However, as you rightly assume, when we are flying dynamically (ie, low levels, BFM, bombing, etc), the AP is disengaged (save autotrim), and the pilot controls the aircraft.

Coming out of combat, as this pilot says: http://youtu.be/ew8NYGY5qnc?t=39m28s

Sure, fighter pilots use autopilot quite a bit. Using an autopilot reduces fatigue on long flights and frees the pilot to navigate, manage systems, and keep a lookout for threats. Disengaging the autopilot is as simple as hitting a button in some aircraft, or just moving the stick in others - it won't slow reaction time or prevent a pilot from responding to some event.

Managing systems sounds a bit funny to me in the context of military jets, yet I imagine that it can take up a large portion of pilot's uptime.

The other answers cover almost everything, but one point that hasn't been mentioned is that the auto-pilot isn't just a labor-saving device, or even a focus-saving device: it reduces human error.
Of course trained pilots, especially combat veterans, can fly extremely reliably, but humans are humans, they can make mistakes and simply engaging the auto-pilot between refueling points on the way to the mission zone and back will get rid of that possibility.


text:
Why are the Antonov An-124 horizontal stabilisers directly behind the wings?
With the wings being on the same plane as the horizontal stabilisers, turbulent air from the wings would interfere with the airflow over the elevators making them flutter. Surely putting the horizontal stabilisers on the vertical stabiliser would be more suitable?

Do you have any source confirming that the AN-124's horizontal stabilizers actually suffer from fluttering?

Does this answer your question: https://aviation.stackexchange.com/a/54318/42636

Flutter is the main reason why the horizontal tail on Antonov designs is mounted low. Many other transport airplanes do indeed sport a T-tail (C-141, Il-76, C-5, C-17, A400M).
Cargo airplanes with a rear ramp suffer from low torsional stiffness of the rear fuselage because of that large cut-out for the ramp doors. If the ramp doors are locked with many locking points around their circumference, this can be remedied but requires those locks to work well and reliably. The Antonov philosophy is to play it safe and assume that Russian maintenance crews will clear the plane even if not all locks engage.
A T-tail adds a significant mass on the lever arm of the vertical tail and lowers the eigenfrequency of the rear fuselage. This brings the torsional frequency of the tail in the range of the first antimetric flutter mode of the wing, as happened on the C-141. Putting the horizontal tail low avoids this and allows for a lighter structure.
Since the wake of the wing moves down with the downwash angle, it passes below the tail of the An-124 in cruise. Not by much, but enough to keep flow over the tail clean. At higher angles of attack the wake will hit the tail, but those should not be encountered in normal operation. If that happens, the main effect is a lower dynamic pressure on the tail, not flutter. For that to happen, you need some coupling of aerodynamic and structural oscillations. For slow flight speed, the wing flaps will bring the wake again well below the horizontal tail.

The horizontal stabilizer on an AN-124 is not directly behind the wings. It is slightly lower than the root of the wings, and the wings have a significant anhedral angle, which effectively eliminates the effect of turbulent airflow from the wings on the stabilizer.


text:
Can a CAP Form 5 flight evaluation be used as the flight portion of a flight review?
I recently completed successfully a Civil Air Patrol Form 5 flight evaluation.  I was told that the Form 5 satisfied the flight portion of the Flight Review.  If true, which additional seminars do I need to complete?

You should have asked your CAP CFI do complete the ground portion as well.  Normally they do.

Welcome to aviation.SE! I reworded your question a little because I think you're asking if it's true that your Form 5 is valid for the flight portion of your flight review. If that isn't accurate then you can simply edit again or roll back my change.

A flight review consists of both a ground portion and a flight portion. You just need a CFI to sign off on the ground training for the flight review.
Alternatively, you could sign up for the FAA Safety Team WINGS program. You get three Flight Activity credits for the above mentioned CAP evaluation. You can earn another three Knowledge Activity credits a number of different ways of your choice. With the three Flight Activity credits and three Knowledge Activity credits, you will complete one phase of the FAASTeam WINGS program. Completing a phase counts as a flight review


text:
What prevents a small plane like a Cessna or Piper from flying as high as a jet?
Obviously taking oxygen in consideration. What prevents a small plane from being able to fly at a much higher altitude? I know some business jets can fly up to 40,000 ft. Does it have to do with the wings air foil? For example, can you fly a Cessna 172 at 20,000 ft or higher?

Hi @Boeing787 - Engine is I imagine important - props don't have big compressors to make the air suitable ... But people have got close - see https://aviation.stackexchange.com/a/32719/56827 or https://aviation.stackexchange.com/a/61899/56827

Just about any Cessna can easily climb to 20,000’ if the engine is equipped with a turbo-charger. The Cessna T210 has a service ceiling of 27,000’. https://en.wikipedia.org/wiki/Cessna_210_Centurion

40000 ft is perfectly normal even for airliners of all sizes. Business jets can go all the way to roughly 50k ft.

Of course the fact the cabin is not pressurised and people have a hard time being conscious at those altitudes without pressurisation is quite a factor. Adding pressurisation has quite a few consequences on the shape, weight and cost of the aircraft.

Your answer implies that jets are large and other aircraft are small. This is not necessarrily the case. Think Hondajet en B-29 :)

What limits a small plane to be able to fly at a much higher altitude?

Money.

Power and lift on an aeroplane both decrease with increasing altitude, as shown in the image above, from my paper copy only old uni book of prof. Wittenberg. At 20,000 ft about 60% of take-off power is still available, at 40,000 ft about 35%.

Solution 1: install a turbo
From Torenbeek, Synthesis of Subsonic Airplane Design, fig. 4-10: maximum altitude of a Piper Navajo, piston engine with and without turbo. The vertical scale is a bit hard to read, it increases with 4,000 ft to a maximum of 24,000 feet. Which the more expensive turbo driven engine reaches, due to compressing the thin intake air at altitude.
Solution 2: install way more power, including multiple turbo's
Install enough power to begin with, and a propeller plane with piston engine can reach 40,000 ft as well, as shown by the data for the P-51 Mustang.

Performance

Maximum speed: 440 mph (710 km/h, 383 kn)
Cruise speed: 362 mph (583 km/h, 315 kn)
Stall speed: 100 mph (160 km/h, 87 kn)
Range: 1,650 mi (2,660 km, 1,434 nmi) with external tanks
Service ceiling: 41,900 ft (12,800 m)
Rate of climb: 3,200 ft/min (16 m/s)
Lift-to-drag: 14.6
Wing loading: 39 lb/sq ft (190 kg/m2)
Power/mass: 0.18 hp/lb (300 W/kg)
Recommended Mach limit 0.8

The service ceiling of 41,900 ft was courtesy of the Packard Merlin engine. Which is not being produced anymore, due to turbine type engines such as in the PC-12 being much lighter and less expensive for the same power rating.The PC-12 reaches 30,000 ft, is still too expensive for the average Joe Private Pilot though.
Conclusion: throw money at it, and it will fly higher...

Not going to downvote a relatively interesting answer, however, I feel I must point out that this doesn't really answer the question. It's not what is the ceiling, it's what creates the ceiling. You start to intimate an answer with "due to compressing the thin intake air at altitude" but don't go into detail.

The Ta 152H had a service ceiling of 49k ft (with nitrous boost), if you want an even more extreme example

@CGCampbell "Power and lift on an aeroplane both decrease with increasing altitude" is the answer.

If an airplane can take off, lift will not limit the altitude it can reach until the Mach effects of the coffin corner kick in. It is really just power that limits the altitude of small planes.

We will take the case of an unsupercharged piston engine, as used in most small Cessnas and Pipers.
All engines need the oxygen in air to burn their fuel. As an airplane climbs, the air thins out and so there's less oxygen available, so the engine can burn less fuel and so it produces less power. There then comes an altitude at which there's not enough power available to pull the plane through the air any faster so as to make more lift in that thinned-out air and keep climbing. Roughly speaking, that is the plane's ceiling.
Now if we put a more powerful engine in the same plane, not only can it climb faster, but there's more power on tap at higher altitude even though its power output is still falling as it climbs. So it can keep pulling the plane through the air progressively faster as it climbs and thereby keep the wing producing enough lift to add altitude. In this way, a more powerful engine will lift the plane higher before running out of power and so exhibit a higher maximum ceiling.
Jet engines are enormously powerful and because of their compressor design, they can make the ambient air denser at high altitudes and so they can maintain their fuel burn rate and keep producing high power up to very high altitudes- far above the altitudes at which a piston-powered plane without a supercharger runs out of climb capacity.
You can add an air compressor (called a turbocharger or supercharger) to a piston engine's intake to trick it into thinking it is breathing sea level air when it is actually at 10,000 feet, and then it can keep climbing, as pointed out by Mike Sowsun- as long as the pilot has an oxygen tank on hand so he or she can stay alive up there.
This is a simplified picture, but not to worry- in a couple of hours, Peter Kaempf will be out of bed and will furnish a more complete reply!

The modern turbo pistons I’ve looked at have a critical altitude of ~18k and ceiling of ~28k, and ~10k is most non-turbo pistons’ ceiling, so that makes sense. Is there a physics limit to how high we can make the critical altitude, or is 18k just a common target due to market factors?

You can go higher if you add more supercharging stages.  WWII fighters had service ceilings up into the 30s and some specialist aircraft with extended wings could get into the 40s.  Problem is, for a civilian airplane you now need to pressurize it. Another limitation is cooling.  The thin air becomes a struggle to get sufficient heat rejection from the cylinders and heat was a big enemy of the air cooled multi-stage supercharged engines above 30000 ft.  It's not impossible but the various limitations make it impractical when jets do the job way better.

@StephenS, there are several factors at work. a plane designed to cruise above 10,000 feet needs an oxygen system and special navigation instruments because it's all controlled airspace up there. O2 and IFR nav aids are very expensive and complicate the business model for small planes. A 18,000 foot service ceiling is more or less typical for an unturbocharged piston plane but that same plane's best-performing altitude will be somewhere between 7,500 feet and 10,000 feet, which coincidentally does not require O2 or IFR instrumentation.

@StephenS: The 18K ft limit is probably because that's the level where everything has to be under positive control and IFR.  10K is NOT the ceiling for most small, non-turbo, piston aircraft.  If it was, there are a lot of mountains hereabouts which you'd hit.  14K is a more reasonable limit.  (Flying between here & the Bay Area, I'd usually use 10,500 or 11,500, depending on direction.  And there are still a few nearby peaks higher than 10,500.)  FWIW, I've flown into airports that were over 7500 ft elevation, for instance Copper Basin in Idaho: http://www.airnav.com/airport/0U2

@CGCampbell It's perfectly normal and acceptable to write ae instead of ä when you can't easily type ä on your keyboard or due to other restrictions. It's the reverse that is wrong, if something is usually spelled ae then you don't use ä.

@CGCampbell Thank you for pointing this out, but oddities like umlauts cannot be insisted on. I don't particularly mind the other spelling - it is used in my email addresses and credit cards due to the inflexibility of those.

Why should I add another answer when yours is already good enough? I would only link to lots of my older answers, like this or this one. I would, however, never say the plane needs to fly faster to produce enough lift for climbing.

Add to this that a plane shaped like a typical Cessna will be near its Mach limit at stall speed not much if any above FL400. This was the limitation on the U-2 -- the airframe was Mach limited, and absolute ceiling was where you couldn't stay above stall at that Mach number.

You can add an air compressor (called a turbocharger or supercharger) to a piston engine's intake to trick it into thinking it is breathing sea level air - There's no trick here, by adding a compressor ahead of the intake you are actually feeding higher pressure air into the engine - no gags or tricks here, the plane thinks it is breathing denser air because it really is.

@ZeissIkon the Cessna has a much lower wing loading than the U2.  A brief check of a TAS calculator shows its Vbg airspeed no problemo at 70000 feet.

@RobertDiGiovanni So you're saying if the powerplant were up to it, you could fly a 152 or 172 to 70k and be below Mach limit for the airframe, but above stall?  TAS doesn't matter for stall, that's IAS -- and Mach goes to lower and lower IAS as the air gets colder...

@ZeissIkon a biplane was taken over 55,000 feet in the 1930s.  Biggest issues were cold and O2 for engine and pilot.  But, with enough thrust to make IAS, a Cessna airframe would fly.  TAS would be around 400 knots for an IAS of 65.  I would add a second wing to the 172 and give it a shot with an electric motor.   60000 feet would be fine.

@RobertDiGiovanni Okay, not quite "in the coffin corner" yet, but not far from it if it takes TAS 400 to stay above stall.  Yep, I was aware of Wiley Post's early pressure suit work (though hadn't understood it was a biplane he flew).  I wonder if anyone has checked the Mach limit for a 172 (or maybe we should use a 175RG).

@PeterKämpf, is the vertical component of thrust in a nose-up climb what is referred to as "hanging the plane on its prop"?

Yes - to show how this is literally true, use a 90° climb which is popular with model airplane builders. In that case, all lift is contributed by the prop. Intermediate climb angles are only linear combinations of a 90° climb and horizontal flight.

The previous answers give an excellent overview of limiting factors regarding high altitude flight with powered civil aviation aircraft like a small Cessna.
Of interest might be that people have operationally used these aircraft at altitudes in excess of the aircraft's original-design flight envelope.  One use in particular which comes to mind has been in special applications such as high-altitude aerial-photo surveys.  A modified Cessna Turbo Stationair has been used in such high-altitude aerial surveys and was flown at altitudes above 37,000 ft (11.3 km).  The plane's engine, modified to have a two-stage turbocharger, could maintain sufficient manifold pressure to fly at such altitudes.
The interior of the plane was essentially stripped of unused, excess-weight items (five of six seats) and other miscellaneous and unnecessary equipment.  The camera bay, aerial camera and camera operating equipment, necessary flight instrumentation and radios, and sufficient oxygen (tanks), were items carried on such flights, along with required fuel.  The owner of this plane commented that at such altitudes, the plane did not fly especially well, but it was manageable.  He also commented on the interesting view looking down on commercial jet aircraft flying at altitude below.
Also of interest might be that other much lighter, essentially smaller, commercially built, single-place, unpowered aircraft have been routinely flown at altitudes well above 40,000 ft (12.2 km). These are sailplanes, and oxygen is required on such flights.  Although generally less than 600 lb (270 kg) in weight (one-fourth the weight of a small Cessna), they have larger span wings (55 ft or about 17 m, more or less) with generally similar wing area compared to a small Cessna powered aircraft. Paul Bickle, retired director of the NASA Dryden Flight Research Facility, flew a Schweizer 1-23E sailplane to an altitude of 46,267 ft (14.102 km) on February 25, 1961.  His flight was within a Sierra wave and was just over 2 hours from take-off to landing.  Bickle's record for altitude gained, 42,303 ft (12.894 km), is unchallenged in a single-place glider.  He suffered outside air temperatures of -65 deg C which frosted the inside of his canopy so badly he could not see the expansive view before him. He only had his instruments to guide his flight, and, because he could not close his cockpit outside-air vents, became so chilled and distracted by the cold that he had difficulty paying attention to piloting his sailplane. Of course, he was more attentive as his canopy cleared in warmer air at lower altitude.  Also, Robert Harris flew a single-place Grob 102 sailplane to a record altitude of 49,009 ft (14.94 km) on February 17, 1986.  His flight was also in a Sierra wave.
In 1952, Larry Edgar and Harold Klieforth set an altitude record in a two-place Pratt-Read G-1 glider, soaring to 44,255 ft (13.489 km) in a Sierra wave. However, on April 25, 1955, Larry Edgar's Pratt-Read glider was destroyed in the lee of the Sierra by a rotor-cloud at 17,000 ft (5.2 km) as he was investigating the rotor's turbulent structure at the base of a wave. The acceleration he experienced, in excess of -20g, ripped off his helmet, boots, gloves, and oxygen mask.  As he drifted downward he could see parts of his glider being carried upward and worried if he pulled his parachute rip-cord, he might be carried upward as well.  Fortunately, he was able to make a parachute landing and survived without breaking any bones. The extreme negative acceleration partially damaged his vision.  Edgar was the only one in his sailplane when this happened.
As one can see, all caution must be used when flying an aircraft in conditions that are not within the original aerodynamic-design flight envelope of the aircraft.  The plane will fly, but nevertheless, aerodynamic control is critical.  This brings to mind the following caution that is especially true in high-altitude flight, and known all too well...
Aviation, although not inherently dangerous, is, to an even greater extent than the sea, terribly unforgiving of any carelessness, incapacity, or neglect.


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How can I find the optimum chord wing length for this wing?
This Aviation SE forum looks the perfect place to ask this question: people here usually make maths the root of most solutions and I think this may solve my aerodynamic optimization problem!
I am having a very hard time optimizing the wing size of my rc plane. I already went to the excelent RC GROUPS community but, despite the generous help of many people, this subject got a little more confusing and personal opinions (practical experience) are conflicting.
My question is this: I already have an rc plane (with tail, fuselage...) and I want to optimize the wing size in order to get efficiency and the longer flight time. I already built dozens of rc planes but this time I want to make this one the most efficient as possible.
IMHO to get longer flight time I need to consume the least amount of energy from my lipo battery as possible - so I need to keep my brushless motor running in the slowest RPM possible, consuming less Amps. Having said that, my total plane weight (not considering the wing, of course) is around 300g (summed weight of ESC, servos, 1000kv motor, 11 inch prop, fuselage, tail, receiver and battery). The weight is fixed, 300g. My wingspan is also fixed, 90cm. I cant have a wing longer than that. The wing shape is rectangular (I know elipses would be the better shape but let's fix the wing shape to simplify this discussion).
If everthing is "fixed" so what is the variable? Well, I want to find the optimum chord wing length for this plane which is gonna bring me the highest flight duration.
I am using the CLARK Y airfoil but you can assume any other one you want. I wouldnt like to discuss airfoil, I would like to discuss the size of the chord and how to optimize it.
My experience and lots of study into this subject says that high aspect ratio wing are usually more efficient than low AR. My wingspan is fixed and all other variables are fixed (to make this discussion more specific) except chord.
So my dear friends, I am struggling a lot to find the optimium chord to get the best flight duration. It looks easy but I am almost 2 months trying to get this answer from many different sources and I still cant find a good solution.
PS: I know I could optimize many of my other fixed parameters, but to avoid having this question marked as too broad/global/generic, I really would like to only optimize the wing chord.
PS (2): the plane that I want to optimize the chord is in the image below. The wing that I show on the picture already provides good flight time (around 22 minutes) but I would like to optimize it in order to get better flight time.

What if you left it as is, but did things to reduce drag? Like add fairing at right angle intersections, rounding the nose, getting rid of the square edges on wing/tail leading edges ...

@CrossRoads thanks for helping, but as I said this plane flies well. Despite that, I am looking to optimize every single part of this plane. But for now I would like to keep things fixed and only optimize chord. After that I will certainly optimize the other things. My plane is far from perfect, so assume my question is related to an already perfect/ideal plane, what would be the optimum chord for this ideal plane?

Try the airfoil tool(s) here,  http://airfoiltools.com/airfoil/index and http://m-selig.ae.illinois.edu/ads.html  Might have to do some reading to find the best shape for gliding overall, and then experimenting with your specific wing length and depth.

@CrossRoads the airfoil database I used a lot, it's a great resource! But in this case I am researching about chord size specifically, not related to any specific airfoil. Assume no airfoil in my question if it would help to provide an answer! :)

I don't know how you seperate airfoil from chord   https://en.wikipedia.org/wiki/Chord_(aeronautics)  Are you just looking for the best distance front to back with a perfectly flat wing?

Maybe the equations here will let you calculate what you need also  https://aviation.stackexchange.com/questions/50597/does-anyone-know-an-equation-relating-wingspan-to-gliding-distance-for-a-glider

What kind of flying do you do with it? Is it slow flight with a few easy turns, fast flight, acrobatic maneuvers, loitering over a small area? Wing choices will be very different for each type of flying.

@TomMcW it is a slow flight, no fast turns, no aerobatics... since I want the longest filght possible.

@CrossRoads The equation that you pointed in the other link does not help much cause it demonstrates that wing aspect ratio relates to efficiency BUT it is not completely true cause a wing with 5 meter wingspan with a chord of 1mm (yes, 1 millimeter) will have a huge aspect ratio number BUT it will never fly, never.

What is the chord on your existing configuration?

@Pilothead my current wing has a chord of about 25cm and it flies well. I would like to know if I should increase it OR decrease it in order to get to the optimum efficiency!

Pretty sure that not bigger is better. Without knowing drag (beyond very) the calculations below suggest slightly smaller, as for a glider. If you cleaned up the aero much smaller would likely be better, as for a sailplane.

@Pilothead your explanation in the other thread I think is the answer I am looking into this thread and the other I opened. I will be glad to accept your answer on both questions cause it is exact what I think is right. However I still didnt understand very well the order of procedures that I should take to find the right chord!

You want to fly at the polar point of the lowest energy consumption. If you describe the drag with the usual parabolic equation, you can use the optimum point derived in this answer. It is
$$3\cdot c_{D0} = \frac{c^2_L}{\pi\cdot AR\cdot\epsilon}$$
Induced drag is three times the parasitic or zero-lift drag when you fly at the polar point for minimum energy loss. But the minimum induced drag at a certain speed is also fixed once wingspan and mass are given - you can only mess up by creating a non-elliptic circulation over span, but never get better than this limit.
Now get the zero-lift drag down as much as you can (pick a thin, highly cambered airfoil which produces a high lift coefficient at the given Reynolds number, definitely no Clark Y) and size chord such that this airfoil will work at the optimum lift coefficient.
Note that this will be an iterative process. Picking a chord will influence your aspect ratio and zero-lift drag, so the optimum lift coefficient will be depend on the chosen chord.

I promise I read your answer almost 10 times and still didnt get it. One thing that I really cant agree is that you told me to get a high cambered profile BUT it generates lots of drag and I think this is not true cause most of the most efficient planes (gliders) are not cambered at all.

@Samul: Those gliders are optimized for best $L/D$; you want minimum sink speed (best $L^3/D^2$). The drag coefficient is higher there, but the actual drag is smaller because of the lower flight speed. And gliders do have camber, some even flaps to increase camber for slow flight. Also, an airfoil with a high lift coefficient allows to use less chord, so there is less surface for the drag coefficient, and it is the product of drag coefficient and surface (times dynamic pressure) that makes the drag force.

thanks for your help. Now I understood better what you said but I still cant get the iterative process. If you could tell more the steps in the iterative process I would more than glad to accept your answer since it looks exactly what I am looking for!

@Samul: It's not that hard. Pick a chord - any chord. Then compute AR and the optimum polar point. Now see where you are: I would expect a $c_L$ somewhere between 1.2 and 1.5 (depending on Reynolds number). If not, repeat. When you get close, select an airfoil that has a good figure of merit ($\frac{c_L^3}{c_D^2}$) and refine your work for that particular airfoil. Generally, a higher $c_L$ without separation will produce the best results.

Since you are looking for maximum endurance, the general configuration you want would resemble a sailplane as far as possible. Wingspan is most important. Aspect ratio really only matters as to how it affects span.
Since you have already constrained weight and span, the next most significant factor is wing loading, directly related to your objective to select chord. Some RC modelers use Wing Cube Loading to determine the appropriate wing area since it works like wing loading but is less affected by scale. Here is a description:
http://www.theampeer.org/M1-outrunners/M1-outrunners.htm#CWL
and here is a calculator
http://www.ef-uk.net/data/wcl.htm
WCL for gliders is less than 4 and up to 8 for sailplanes. Using your 300g weight constraint translates to 12-18dm2 of wing area. Dividing by your 9dm span constraint means chord should be in the range 1.3-2dm.

thanks, nice links you provided (even the calculator is great). However I would like to know the ideal/optimal chord (actually I would like to know the theory/formulas used) not a range. Imagine this: if I plot a graph of EFFICIENCY x CHORD then I am pretty sure that the graph will be someting like f(x) = -x^2 where there will be an optimum point where the chord gets the most efficiency posible.

I doubt there is any such graph that does what you ask because aircraft are not designed a single parameter at a time. The interaction of lift, drag and other parameters have much more effect on the result than a few percent difference in chord. Your current construction methods cause considerable drag, so I would guess that the glider sized wing would be preferable. If you cleaned up your aero (at least the square leading edges) and carried an airfoil to the tip the sailplane size would likely be significantly better.

I intend to do the optimizations you rightly said here, but first I want to get the chord right. When you reply me in the other thread I think I will get what you mean and I am pretty sure that's in the right direction!

My final year design project for my undergrad (mechanical) looked at designing a UAV to get the best lift to drag ratio under several constraints (wingspan, max weight, etc.) set by our advisor. One of the tools we used to find the optimum reference area (S) for our preliminary design is a matching chart. An example of one is shown below:

(source:   http://www.fzt.haw-hamburg.de/pers/Scholz/dimensionierung/intro.htm)
The matching chart plots Thrust to weight ratio (T/W) against your Wing Loading (W/S). The lines you see are obtained by plotting your performance constraints.
Any point on the hatched region does not meet your requirements.
The match point shown in the picture gives you your optimum T/W and W/S according to your inputs. This is the point where your T/W and W/S are at minimum, while still meeting your requirements.
From your match point you can get your reference area if you know the total weight of your plane.
Once you know your area, you can get your desired chord from your span constraint.
Snorri Gudmunsson's textbook, General Aviation Aircraft Design - chapter 3, has a section on how you can make a matching chart.

thanks for helping. After you teaching me about this "matching chart". I searched a lot about it on google but I still couldnt find a way to mathematicaly optimize the chord (or Area as you stated). I am amazed by how incredible important is this question that I am asking and the total lack of content online to help with that. I think this shouldnt be that hard, I just want to optimize one single parameter: chord. Do you know why is it so hard to find a bibliography about this optimization?

Another option is to model your plane on XFLR5 and play around with the different chords until you obtain your desired performance. There's plenty of tutorials on youtube on how to use XFLR5. I guess there's not much bibliography because the optimization really depends on what type of flying you want to do (i.e. acrobatics or light flying)

XFLR5 is great but I dont know how to translate the analysis data into "efficiency". The lift/drag diagram is great but if I compare two exact planes with different chords, how will I know which one is better?

Maybe you can get something from the Daedalus:

The MIT Aeronautics and Astronautics Department's Daedalus was a
  human-powered aircraft2 that, on 23 April 1988, flew a distance of
  72.4 mi (115.11 km) in 3 hours, 54 minutes, from Iraklion on the island of Crete to the island of Santorini. The flight holds official
  FAI world records for total distance, straight-line distance, and
  duration for human-powered aircraft....  Airfoils Mark Drela had
  written the program XFOIL which enables the design of aerofoils and
  which can accurately predict the performance at any Reynolds number
  which is actually superior to a wind-tunnel test of a proposed
  section. The wing sections never existed on paper, either as lists of
  numbers or as drawings. The information came out of the design program
  onto disc, and this guided the cutter. The spar of the MLE (Michelob
  Light Eagle) was three tubes aligned vertically.

Source

This scheme was abandoned for the final Daedalus design where the more
  usual single tube was used. Construction of Light Eagle took 15,000
  hours work by 18 members of MIT. The airfoils which were used are:
  DAE11, DAE21, DAE31, and DAE51.

Wish I knew what tags or whatever are being used to create the quotes and show the images like that, I'd try and start using them more.

You can click "edit" and see the markdown, or use the buttons at the top of the edit box.

@CrossRoads thanks for researching and bringing this plane in but actualy it does not help me to discover the best chord possible. It is indeed a great performance plane but I have fixed contrainsts that I would like to keep fixed. BTW it was not me who downvoted.

Here's a link to some of the basic markdowns. I have it bookmarked and refer to it all the time.

Thanks TomMcW -  I have bookmarked that now also.


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What is the lowest-speed bogey a jet fighter can intercept/escort?
Jet fighters are made to cruise at a high subsonic speed, even at a supersonic speed (supercruise?). It means that they can be flown comfortably at relatively high speeds.
Jet fighters are often used to intercept aircraft, whatever the reason (assistance, escort from national to international airspace, check an unresponsive aircraft,...).
General aviation aircraft can cruise at a slow speed compared to jet fighter. Moreover, if needing assistance, it may fly even slower. I imagine it is possible that a GA aircraft needing assistance cannot fly at a speed above the stall speed of a jet fighter. I know that some countries dispatch choppers to intercept slow aircraft to handle such situation.
I imagine the military doesn't fly their jets at stall speeds when close to another aircraft (a margin is needed).
Is there a speed below which a jet fighter cannot intercept another aircraft? Otherwise, how do they do it? (I imagine you can orbit above another aircraft but it is less convenient to guide it safely to an airport)

Edited title for typo - hope that’s what was intended!

There was an episode of The Simpsons where Sideshow Bob tried to escape in the Wright Flyer. (Yes, the first airplane). It was so slow that the jet pilots who went after him had to land their planes and run after him on foot.

There is nothing stopping a faster jet from flying a zigzag pattern behind an intercept target if the goal is to remain in close contact with it, but a large difference in airspeeds would indeed hamper some tasks like observing the cabin for signs of life or unlawful interference. So the answer is yes, a jet fighter cannot perform all the tasks involved in a typical interception of an unresponsive aircraft if the speed delta is too high.
For reference, the takeoff speeds of 4.5 generation fighters like the F-16 are in the 130 kt (240 km/h) region, while the takeoff speed of something like a C172 is around 60 kt (111 km/h). The exact weapon and fuel load of the fighter will of course have an impact, but fighters scrambled for an intercept are typically light and with an almost clean configuration. I have deliberately used takeoff speeds instead of landing speeds to factor in a margin of safety in regards to control authority; both speeds are among the lowest an aircraft can sustain, but during  close approaches to a potentially hostile aircraft, some maneuverability is required.
Still, 100+ km/h is a substantial speed gap, and has led to issues in the past:

The Po-2 is also the only biplane credited with a documented jet-kill, as one Lockheed F-94 Starfire was lost while slowing down to 161 km/h (100 mph) – below its stall speed – during an intercept in order to engage the low flying Po-2.

Presumably Starire -> Starfighter. | The SF didn't need any reason to fall out of the sky - the Polykarpov was just an excuse. |||| Q:  "How do you acquire a Starfighter?" A: "Buy a plot of land. and wait."

In WW2 Po-2s "killed" 109s the same way

@RussellMcMahon To be honest I was surprised it wasn't a Starfighter. It would have been a fitting cherry on top of its nefarious record.

No, I'd install the U2 wings on the Starfighter, then chase it.

Military fighter jets are actually not all that fast, when they aren't supersonic. Their cruise speeds are below the cruise speed of airliners. Supersonic flight is mostly meant for interception, while combat speeds are subsonic. The best-range cruise speed for most fighters in the range of 300 knots, due to high drag.
Below their cruise and maneuvering speeds, there is considerable margin to an actual stall. It depends on weight and altitude, so there is no one answer, but it goes down to 100 knots and below.
The most popular GA aircraft, the Cessna 172, has an official cruise speed of 122 knots. This is within the operating limits of most jet fighters, except when overloaded or at altitude above that of GA aircraft. So there is a sufficient amount of overlap between GA and jet speeds, if both want to stay alongside.
However, a GA aircraft can operate much slower, and jet fighters generally won't be able to maintain a steady position close to their stall speeds. While their TWR can be close to 1, the typical fighter aircraft is simply not controllable at a 45-degree angle of attack. Make no mistake, a fighter won't lose a GA plane, but it won't be able to maintain a steady position next to it.
Fighters with thrust vectoring, which are still rare, could sustain mostly side-by-side contact with a GA aircraft. The Pugachev's Cobra maneuver can slow a supermaneuverable plane to near-zero airspeed, although it will have to get back to its stall speed after.
That said, a helicopter is still more practical for assisting a GA airplane, not least because it's much more likely to be able to land near the plane's crash site to help pull the occupants out.

Wow, the old straight-tail 172 I flew sure didn't cruise at 122 knots.  Anyway, would almost be worth busting some Russian airspace just to see someone doing a Cobra maneuver next to me.

@Therac, I downvoted your answer because I don't agree with the first paragraph.  True that the slower the speed the tighter the turn, but no fighter will enter a merge at 200 knots.  Speed is life.  There is a number called "cornering speed" where the aircraft is fast enough to pull right up to the G limit without entering an accelerated stall, but will stall before overstressing.  This is where the best turn performance is achieved.  I didn't fly fighters, but in the Prowler this was 300KIAS for a frame of reference.  Still, we practiced defensive ACM much faster.

@MichaelHall Ouch, re-read my post - I merged two sentences I shouldn't have.

I still don't like the answer.  With flaps down at 120 a fighter can make a radius of turn equivalent to a Cessna at cruise.  However, it isn't about the radius, in a fight you want the best turn rate.  A better turn rate will generate the angles to achieve an advantage and take a shot.  Best rate occurs at cornering speed.  I will have to ask an old Hornet friend or do a search on what this typically is for a fighter.  I also think cruise is closer to 420 TAS or .7 mach.  It was for us...

@MichaelHall I've read corner speed for an F-16 is around 450 kts. Not sure how typical that number is among fighter aircraft.

@Todd Wilcox, I would believe that over 200.  Thanks.

Depending on aircraft type and weight, flaps down approach speeds for most military fighters is in the range of 120 to 150 KIAS.  This is within the speed range of medium to higher performance GA aircraft, but a bit too fast for most fixed pitch prop, fixed landing gear aircraft in common use.  Some fighters are quite controllable at high AoA at speeds lower than optimal for approach, but it is unlikely that a fighter pilot would be comfortable flying formation at such a slow speed.
Additionally, there is almost no assistance a military jet could provide to a GA aircraft that would require the two to be in close formation at matching airspeed.  The nearest situation necessitating actual close formation flight would be to provide visual verification whether landing gear were fully extended.  While close up inspection of an over-center lock is possible to confirm, a general up, down, or stuck halfway assessment is possible from quite a bit further.
Even if it were ideal or possible, unbriefed formation flight between dissimilar aircraft with a GA pilot of unknown proficiency presents its own set of risks.  Communication relay to ATC, assigning headings or leading a lost-comm pilot to a hole in the clouds, etc. are legit forms of assistance that could be provided from an interceptor orbiting higher overhead, or leading on at a slightly higher airspeed.

The most useful assistance a military aircraft could provide to a GA aircraft is guidance if the later lost comms over unfamiliar terrain or cannot be guided over radio due to lack of radar coverage. And that does require the guiding aircraft to fly slow enough that the guided one can keep up with it.

I fail to see how it answers the question (what is the slowest-speed?)

@Manu, you are correct, I didn't directly answer the question with a specific airspeed.  My point is that you don't have to match speeds to intercept, and you wouldn't likely escort in close formation either.

@MichaelHall You should edit your answer to explicit this point (no lowest speed as you don't need to match bogey's airspeed), otherwise you answer may be flagged as not an answer.

@Manu H - edited per your suggestion.  Please let me know what you think.